Extreme variations in strontium initial ratios in ore-related fluids

Abstract

Magmato-hydrothermal cassiterite-topaz ore at the Carboniferous East Kemptville (EK) greisenhosted tin deposit (Nova Scotia, Canada) is cross-cut by veins containing apatite, triplite, vivianite and fluorite. Initial 87Sr/86Sr ratios of these minerals have an extreme range (0.7135 to 0.8284). The initial ratios of the host rocks, EK quartz-topaz rock and Davis Lake biotite monzogranite (0.729±0.001, 0.727±0.004), are also high. The adjacent Meguma Group metasedimentary rocks are more typical of crustal material (0.712–0.719 at 330 Ma). Rb and Sr contents of EK fluorite (max.: 13.0, 1420 ppm) and apatite (max.: 88.1, 6660 ppm) are unusually high and variable. Unexpectedly, high Sr contents correlate positively with the high initial 87Sr/86Sr ratios. Fluorite and phosphate minerals from the first set of post-greisen veins at East Kemptville reflect the unusual chemistry of a high 87Sr/86Sr fluid present in the deposit after ore formation. The most extreme composition of this fluid was characterized by 87Sr/86Sr>0.8284, high Rb/Sr, high P, Rb, Cu, Zn and Fe contents, but low abundances of Ca, Pb and Sn with respect to the Davis Lake monzogranite. Such a fluid could have been derived from the greisen fluid and modified by reaction with the overlying Meguma metawacke. A second alternative, which cannot be well constrained at present, is that an extremely radiogenic fluid entered the deposit after ore formation and mixed with the postore fluid. In either case, the modified fluid subsequently mixed with meteoric water and precipitated the minerals with much lower 87Sr/86Sr ratios and Rb and Sr contents.